The subject matter described herein pertains to the automatic measurement of the evaporation of water from a surface that mimics the albedo and diffusion resistance to water vapor of the leaf of a plant. As a class, devices of this sort are called atmometers. The instant device whose method and apparatus are disclosed may be termed an automatic atmometer.
Prior art atmometers have to be read manually, generally by making visual note of the fluid level against a sight gauge. This is both bothersome and error prone. It would be desirable if the readings could be taken electronically, even in the absence of an observer, and with a degree of accuracy exceeding that afforded by sight gauges.
These objectives are achieved in an automatic atmometer whose evaporating head draws its fluid through a three-way valve whose quiescient position connects the evaporating head to one end of a metering vial equipped with fluid level sensors indicating "full" and "empty". The fluid metering vial is open to the atmosphere at its other end. The difference in volume contained by the metering vial when full and when empty is its displacement. The displacement is selected to be fairly small, say, a milliliter.
Evaporation of water at the evaporating head draws water by suction out of the metering vial. When the sensors indicate that the vial is empty the three-way valve is activated. This blocks the connection to the evaporating head and (temporarily) connects the metering vial to a fluid reservoir located above it. Gravity flow refills the metering vial until the sensors indicate that it is full. At that time the three-way valve is returned to its quiescient condition.
Each refilling of the metering vial indicates that an amount of water equal to the displacement of the metering vial has been evaporated. The circuit that controls the switching of the three-way valve also produces an output indicative of that switching. That output can be used to increment or otherwise signal a recording or other data logging device.
The subject matter disclosed herein also pertains to a method and apparatus for the measurement of fluid flow with a metering vial equipped with capacitive sensors to detect the presence and absence of fluid at various places along the vial. The metering vial is intended to function reliably in environments where high surface tension and the possibility of condensation combine to produce the possibility that droplets of condensation might be sucked back up into a distal end of the vial open to the atmosphere. It is desirable for the method and apparatus for measuring fluid flow to work with equal accuracy for fairly rapid flows, as well as for flow rates that are almost zero.
These objectives are met by a metering vial having an inlet end below an expansion chamber, above which is a U-shaped siphon extending downwards on the other end by a convenient amount. A fluid detector is located proximate the inlet, while two fluid detectors are located some distance apart along the downward extending leg of the siphon leading to the opening to the atmosphere. The inlet fluid sensor detects, by the absence of fluid, that the metering vial is empty. The other two fluid sensors, by their simultaneously detecting the presence of fluid, indicate that the vial is full. A logic circuit controls whatever response is desired to these detected conditions.
The expansion chamber has a volume exceeding that portion of the metering vial that is between the two fluid sensors that cooperate to indicate the full-of-fluid condition. This volume relationship property, in conjunction with the logical AND'ing of the outputs of those two sensors, operates to produce immunity to the problem of condensation and surface tension. This is achieved by avoiding a false full indication that might occur if a drop of condensation were sucked into the open end of the metering vial as the vial's displacement is drawn out the inlet end.
Each capacitive fluid sensor includes a driving plate and a driven plate, separated by a grounded shield. Each plate is essentially a cylinder around a section of glass tubing that is part of the metering vial. The shield is a metal plate with a hole therein to allow passage therethrough of the glass tube. The driving plate is connected to the output of a high level radio frequency oscillator. The amount of signal coupled through the capacitor varies greatly according to the dielectric constants of the material between the plates. When there is no fluid present there is only the relatively low dielectric constants of the glass tubing and the air therein. The shield blocks direct ("line of sight") coupling between the plates. However, when there is fluid in the tube the dielectric constant is increased, and a greater signal is coupled into the driven plate. The signal at the driven plate is rectified and compared to a reference voltage to produce a logic signal indicative of the presence or absence of the fluid.
The reference voltage is produced from another capacitor constructed in the same general fashion as those for the sensors. This gives the reference voltage the same temperature coefficient as the sensors.